Methods and apparatus for thread-based scheduling in multicore neural networks

ABSTRACT

Systems, apparatus, and methods for thread-based scheduling within a multicore processor. Neural networking uses a network of connected nodes (aka neurons) to loosely model the neuro-biological functionality found in the human brain. Various embodiments of the present disclosure use thread dependency graphs analysis to decouple scheduling across many distributed cores. Rather than using thread dependency graphs to generate a sequential ordering for a centralized scheduler, the individual thread dependencies define a count value for each thread at compile-time. Threads and their thread dependency count are distributed to each core at run-time. Thereafter, each core can dynamically determine which threads to execute based on fulfilled thread dependencies without requiring a centralized scheduler.

PRIORITY

This application is a divisional of, and claims the benefit of priorityto U.S. patent application Ser. No. 17/367,521 filed Jul. 5, 2021 andentitled “METHODS AND APPARATUS FOR THREAD-BASED SCHEDULING IN MULTICORENEURAL NETWORKS”, that claims the benefit of priority to U.S.Provisional Patent Application Ser. No. 63/050,090 filed Jul. 9, 2020and entitled “METHODS AND APPARATUS FOR LOCALIZED PROCESSING WITHINMULTICORE NEURAL NETWORKS”, each of which are incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Agreement No.N00014-19-9-0003, awarded by ONR. The Government has certain rights inthe invention.

RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.17/367,512 filed Jul. 5, 2021 and entitled “METHODS AND APPARATUS FORLOCALIZED PROCESSING WITHIN MULTICORE NEURAL NETWORKS”, and U.S. patentapplication Ser. No. 17/367,517 filed Jul. 5, 2021 and entitled “METHODSAND APPARATUS FOR MATRIX AND VECTOR STORAGE AND OPERATIONS”, each ofwhich are incorporated herein by reference in its entirety.

COPYRIGHT

A portion of the disclosure of this patent document contains materialthat is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent files or records, but otherwise reserves all copyrightrights whatsoever.

TECHNICAL FIELD

This disclosure relates generally to the field of neural networking.More particularly, the present disclosure is directed to hardware,software, and/or firmware implementations of neural network processing.

DESCRIPTION OF RELATED TECHNOLOGY

Incipient research is directed to so-called “neural network” computing.Unlike traditional computer architectures, neural network processingemulates a network of connected nodes (aka neurons) that loosely modelthe neuro-biological functionality found in the human brain. Whileneural network computing is still in its infancy, such technologiesalready have great promise for e.g., compute rich, low power, and/orcontinuous processing applications.

Existing neural networks are most commonly emulated withingeneral-purpose programming environments because commodity hardware andsoftware compilers are well understood and readily available.Unfortunately, such implementations suffer from many inefficiencies dueto e.g., hardware limitations (e.g., physical connectivity), compilerdesign, and/or instruction scheduling. Neural networks would be a greatfit for parallel processing and distributed computing models; however,corresponding changes to hardware and compilers are needed.

SUMMARY

The present disclosure addresses the foregoing needs by disclosing,inter alia, methods, devices, systems, and computer programs forthread-based scheduling within multicore network processors.

In one aspect, systems, methods, and apparatus for thread-basedscheduling are disclosed. One exemplary apparatus includes: a scoreboardconfigured to store thread dependency counts for one or more threads; aready queue configured to identify unblocked threads of the one or morethreads; an instruction fetch/instruction decode lane configured tofetch one or more instructions corresponding to an execution thread; anissue controller configured to issue the one or more instructionscorresponding to the execution thread to an execution unit; theexecution unit configured to perform operations based on the one or moreinstructions corresponding to the execution thread; and where a firstinstruction of the one or more instructions updates the threaddependency counts for the one or more threads. In a related embodiment,a method to perform the foregoing techniques is described. In oneexemplary embodiment, the method includes storing thread dependencycounts for one or more threads; identifying unblocked threads of the oneor more threads; fetching one or more instructions corresponding to anexecution thread; issuing the one or more instructions corresponding tothe execution thread to an execution unit; performing operations basedon the one or more instructions corresponding to the execution thread;and updating the thread dependency counts for the one or more threads

In one aspect, systems, methods, and apparatus for thread-basedscheduling are disclosed. One exemplary method includes: monitoringthread states of a thread pool in a control path logic of a core of themulticore processor, where the thread states of the thread pool arespecific to the core of the multicore processor; determining when afirst thread of the thread pool is ready for execution; assigning aprocessor resource to the first thread; monitoring a utilization of theprocessor resource by the first thread; and updating a first threadstate of the thread pool in the control path logic. In a relatedembodiment, apparatus configured to perform the foregoing techniques isdescribed. One exemplary apparatus includes: logic to monitor threadstates of a thread pool in a control path logic of a core of themulticore processor, where the thread states of the thread pool arespecific to the core of the multicore processor; logic to determine whena first thread of the thread pool is ready for execution; logic toassign a processor resource to the first thread; monitoring autilization of the processor resource by the first thread; and logic toupdate a first thread state of the thread pool in the control pathlogic.

In one aspect, systems, methods, and apparatus for thread-basedscheduling are disclosed. One exemplary multicore processor includes: afirst core and a dedicated first control path logic configured to storea first set of thread states; a second core and a dedicated secondcontrol path logic configured to store a second set of thread states;and at least one router configured to asynchronously connect the firstcore to the second core, where the first core comprises at least oneinstruction, that when executed by the first core, synchronously updatesthe first set of thread states of the first core, independently of thesecond core. In a related embodiment, methods configured to perform theforegoing techniques is described. One exemplary method includes:storing a first set of thread states; storing a second set of threadstates; asynchronously communicating between cores; and synchronouslyupdating the first set of thread states, independently of the second setof thread states.

Other features and advantages of the present disclosure will immediatelybe recognized by persons of ordinary skill in the art with reference tothe attached drawings and detailed description of exemplary embodimentsas given below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of a multicore processorarchitecture, commonly used within the processing arts.

FIG. 2A is a graphical representation of one exemplary multicorearchitecture, in accordance with the various principles describedherein.

FIG. 2B is a graphical representation of the extensible nature of themulticore architecture, in accordance with the various principlesdescribed herein.

FIG. 3 is a logical block diagram illustrating the data traffic flowthroughout the multicore architecture, in accordance with the principlesdescribed herein.

FIG. 4 is a logical block diagram of an exemplary core's control anddata path operations, in accordance with the principles describedherein.

FIG. 5 is a logical block diagram of one exemplary thread dependencygraph between two cores, in accordance with the principles describedherein.

FIGS. 6A-6F are graphical representations of an exemplary core's controland data path operations, in accordance with the principles describedherein.

FIG. 7 is a logical flow diagram of a generalized method forthread-based scheduling, in accordance with the principles describedherein.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof wherein like numeralsdesignate like parts throughout, and in which is shown, by way ofillustration, embodiments that may be practiced. It is to be understoodthat other embodiments may be utilized, and structural or logicalchanges may be made without departing from the scope of the presentdisclosure. Therefore, the following detailed description is not to betaken in a limiting sense, and the scope of embodiments is defined bythe appended claims and their equivalents.

Aspects of the disclosure are disclosed in the accompanying description.Alternate embodiments of the present disclosure and their equivalentsmay be devised without departing from the spirit or scope of the presentdisclosure. It should be noted that any discussion herein regarding “oneembodiment”, “an embodiment”, “an exemplary embodiment”, and the likeindicate that the embodiment described may include a particular feature,structure, or characteristic, and that such particular feature,structure, or characteristic may not necessarily be included in everyembodiment. In addition, references to the foregoing do not necessarilycomprise a reference to the same embodiment. Finally, irrespective ofwhether it is explicitly described, one of ordinary skill in the artwould readily appreciate that each of the particular features,structures, or characteristics of the given embodiments may be utilizedin connection or combination with those of any other embodimentdiscussed herein.

Various operations may be described as multiple discrete actions oroperations in turn, in a manner that is most helpful in understandingthe claimed subject matter. However, the order of description should notbe construed as to imply that these operations are necessarily orderdependent. In particular, these operations may not be performed in theorder of presentation. Operations described may be performed in adifferent order than the described embodiment. Various additionaloperations may be performed and/or described operations may be omittedin additional embodiments.

The Complexity of Software-based Neural Networks

FIG. 1 is a graphical representation of a multicore processorarchitecture 100, commonly used within the processing arts. Themulticore processor 102 may include one or more cores 112A, 112B . . .112N. Each core may include logic (e.g., arithmetic logic units (ALUs),registers, etc.) arranged to perform various control and data pathoperations. Examples of control and data path operations may includewithout limitation: instruction fetch/instruction decode (IF/ID),operation execution and addressing, memory accesses, and/or data writeback. A small amount of frequently used instructions and data may belocally cached “on-chip” for fast access; otherwise, “off-chip” storageprovides cost-effective storage of bulk data (104A, 104B . . . 104N).

During operation, the processor cores 112A, 112B . . . 112N read andwrite computer instructions and/or data from the external memories 104A,104B . . . 104N via a shared bus interface 106. Each computerinstruction (also referred to as an “opcode”) identifies the operationto be sequentially performed based on one or more operands (data,register locations, and/or memory addresses). By linking togethersequences of computer instructions, it is possible to compute anycomputable sequence.

In “general-purpose” computing, the processor cores and memories may betasked with any arbitrary task. A shared bus architecture and monolithicmemory map flexibly allows every core 112A, 112B . . . 112N to accessany memory location within the external memories 104A, 104B . . . 104N.As a practical matter, however, the shared bus interface 106 isphysically pin-limited; there is a fixed width data bus that servicesall processor-memory connections one-at-a-time. Limited connectivity cansignificantly affect performance where multiple cores try to access thememories at the same time. Additionally, local cache sizes are limited;reading and writing to large data structures may require multiple“off-chip” transactions across the pin-limited bus. Finally, “global”data structures cannot be accessed by more than one core at a time(simultaneous access could result in data hazards and race conditions).

Unlike general-purpose computing, so-called “neural network” computinguses biologically-inspired algorithms that take their inspiration fromthe human brain. Neural networks are characterized by a multi-layeredcomposition of high-dimensional linear and non-linear functions. Theintermediate function outputs between layers are known as activations.Neural networks typically contain a large number of parameters that areused for e.g., vector-matrix operations. The parameters are tuned in agradient descent training process based on known input/output datapairings. After training, the parameters are held constant duringdeployment as the neural network processes novel input data to executeits trained task. For example, FIG. 1 graphically depicts one exemplaryneural network computation that is performed as a vector-matrixmultiplication 150. As shown therein, neural activations are modeled asa vector of digital values (a) that are multiplied by a matrix ofparameter weights (B) for the neural network; the output (c) correspondsto the output neural activations.

Unfortunately, naïvely allocating neural network processing to themulticore processor architecture 100 is extremely inefficient. Firstly,each of the cores 112A, 112B, . . . 112N must access the complete set ofneural network data structures. The vector and matrix dimensions are afunction of the number of nodes (neurons) within the neural network,thus neural networks of any significant size exceed data sizes that canbe efficiently cached on-chip. As a result, all of the cores 112A, 112B,. . . 112N constantly move data across the pin-limited bus interface106. Additionally, each of the cores 112A, 112B, . . . 112N read andwrite to the same data structures (a, B, c) and often block one another.

As a related issue, “Big O” notation is used in the computer arts toclassify algorithms according to computational complexity (run time andspace requirements O, as a function of input size N.) Big O notation iswidely used to describe the limiting behavior of a function as itincreases, e.g., processing complexity, memory storage, bandwidthutilization, etc. For example, vector-matrix multiplication has acomputational complexity of O(N²) for vector size (N) because eachelement of the vector must be multiplied by a corresponding element ofeach row and column of the matrix. Doubling the vector size (N)quadruples the computational complexity (O(N²)).

Referring back to FIG. 1 , existing neural networking solutions rely ongeneral-purpose vector-matrix operations. Such solutions often rely onhardware accelerators to perform “brute-force” element-by-elementcalculation. However, the data structures that are used in neuralnetwork processing can be made to be quite sparse (a high ratio of nullvalues.) Brute force vector-matrix operations can be particularlyinefficient for sparse data structures because the vast majority ofmemory reads, vector-matrix multiplications, and memory write-backs areunnecessary (null valued). Furthermore, as neural networks continue togrow in size and complexity, inefficient brute force solutions willquadratically increase in complexity.

Substantial factors in neural network energy consumption may includemoving large amounts of data, and storing a large number of parametersin leaky SRAM (static random access memory). Charging and dischargingwires to transfer data takes energy. Wire energy costs scale with wirelength (e.g., chip area) and is a significant concern for chip design.As a related issue, neural networks are parameter-rich, but on-chip SRAMmemory is costly to implement. On-chip SRAM is optimized forperformance, not power consumption, so SRAM cells may consumesignificant amounts of energy even when idle, due to leakage. Thecombination of these factors can limit neural network adoption; in onespecific example, remote applications are often power constrained.

Exemplary Multicore Architecture

The aforementioned complexities of neural network processing havepresented significant issues for embedded device implementations.Notably, existing neural network implementations are handled withinsoftware, without regard to the underlying hardware platformlimitations; unfortunately, physical connectivity (e.g., pinlimitations), computational complexity, and/or scheduling overheadpresent significant obstacles for embedded devices. More directly,improved solutions for handling neural networks in embedded environmentsare needed; ideally, such solutions should enable compute rich, lowpower, and/or continuous processing applications.

To these ends, various principles described herein synergisticallyleverage locality, sparsity, and distributed scheduling, to enableneural network processing within embedded hardware applications. Unlikeexisting solutions that rely on commodity software and hardware toperform “brute force” large scale neural network processing; the varioustechniques described herein map and partition a neural network based onthe hardware limitations of a target platform. The exemplaryhardware-aware mapping/partitioning described herein enhances neuralnetwork performance by e.g., avoiding pin-limited memory accesses,processing data in compressed formats/skipping unnecessary operations,and distributing task scheduling while decoupling timing requirementsbetween cores.

FIG. 2A is a graphical representation of one exemplary multicorearchitecture 200, in accordance with the various principles describedherein. As shown, the architecture 200 does not use an external memoryto store the neural network data structures nor any intermediateresults. Instead, each core includes its own processing hardware (212A,212B, 212C, 212D), local weights (214A, 214B, 214C, 214D), globalweights (216A, 216B, 216C, 216D), working memory (218A, 218B, 218C,218D), and accumulator (220A, 220B, 220C, 220D). While the followingdiscussion is presented in the context of a core with its own dedicatedmemories, the techniques described herein may be used in shared memorysystems and/or hybrids thereof. More generally, dedicated core resourcesmay enable improved core performance whereas shared resources acrosscores may provide flexibility and/or cross-core communicationopportunities.

As shown in FIG. 2A, the local neighborhood weights are stored in thelocal weight memories (214A, 214B, 214C, 214D) and each core's subset(or “slice”) of the global network weights are stored in the globalweight memories (216A, 216B, 216C, 216D). During operation, applicableweights are retrieved from the corresponding memory for computation;intermediate results may be stored within a working memory (218A, 218B,218C, 218D) and/or accumulator (220A, 220B, 220C, 220D).

While the illustrated embodiment is shown in the context of four (4)cores emulating a global neural network of nodes, the multicorearchitecture described herein may be broadly extended to any number ofcores and/or any number of nodes (see e.g., FIG. 2B). Additionally, theforegoing discussion presented a symmetric distribution, howeverasymmetric distributions may be substituted with equal success.Partitioning may be scaled to individual core's capabilities and/orapplication requirements. For example, asymmetric systems may enablehigh performance cores (more logic, memory, and/or faster clock rates)and low power cores (less logic, less memory, and/or power efficientclocking). In such implementations, matrix operations may be sized so asto complete within operational constraints, given a core's capabilities.Furthermore, any consolidation, division, distribution, agglomeration,and/or combination of processing hardware and/or memory may besubstituted by artisans of ordinary skill in the related arts, given thecontents of the present disclosure.

FIG. 3 is a logical block diagram illustrating the data traffic flow 300throughout the multicore architecture, in accordance with the variousprinciples described herein. Each neighborhood (302A, 302B, 302C, 302D)is characterized by a locally dense neural network. Neighborhoods areconnected via a global interconnect matrix (304A, 304B, 304C, 304D) tothe other neighborhoods; the output of the neighborhoods can be furthersparsified prior to global distribution via interconnect logic (306A,306B, 306C, 306D).

Unlike existing neural network processors which naïvely distributeprocessing load (discussed supra), the exemplary multicore architecturedecouples processing among the cores. For example, as discussed in U.S.patent application Ser. No. 17/367,512 filed Jul. 5, 2021 and entitled“METHODS AND APPARATUS FOR LOCALIZED PROCESSING WITHIN MULTICORE NEURALNETWORKS”, previously incorporated herein by reference in its entirety,the neural network processing may be mathematically transformed (mapped)and spatially partitioned into dense “neighborhood” processing andsparse “global” communications processing. The mapping/partitioningpreserves the properties of the original global neural network at afraction of the memory accesses.

In one such embodiment, a global neural network is mapped into a set ofsparsely interconnected, dense neighborhood neural networks that arepartitioned based on hardware platform constraints. The transformationmay be performed on a modified gated recurrent unit (GRU). Alternativeimplementations may perform the transformation on modified LongShort-Term Memory (LSTM) or any other “remember-forget” recurrent neuralnetwork (RNN) logic. More generally, any logic or component thatretains/removes information between nodes of the neural network may bemodified to transform a first domain (first vector space) to a seconddomain (second vector space). By avoiding data path dependencies betweencores, the exemplary multicore architecture can maximize parallelism tocomplete tasks quickly and maximize the time spent in low-power sleepstates to mitigate leakage. In one embodiment, each core is avariable-length Single-Instruction-Multiple-Data (SIMD) machine that canexecute the same operation (instruction) on multiple data elements inparallel (e.g., a matrix-vector multiply or a pointwise nonlinearity ona vector). Data paths operate in parallel, so multiple instructions canexecute simultaneously in a core. Likewise, cores operate in parallel,communicating only when necessary.

As a further optimization, there are overhead costs associated withcompression, and different techniques have different costs and benefits.Since vectors and matrices are used differently in neural networkprocessing, these data structures may be represented differently tofurther enhance performance. For example, as discussed in U.S. patentapplication Ser. No. 17/367,517 filed Jul. 5, 2021 and entitled “METHODSAND APPARATUS FOR MATRIX AND VECTOR STORAGE AND OPERATIONS”, previouslyincorporated herein by reference in its entirety, exemplary embodimentscompress sparse neural network data structures based on actual,non-null, connectivity (rather than all possible connections). Thisgreatly reduces storage requirements as well as computationalcomplexity. In some variants, the compression and reduction incomplexity is sized to fit within the memory footprint and processingcapabilities of a core. The exemplary compression schemes representsparse matrices with links to compressed column data structures, whereeach compressed column data structure only stores non-null entries tooptimize column-based lookups of non-null entries. Similarly, sparsevector addressing skips ed entries to optimize for vector-specificnon-null multiply-accumulate operations.

Additionally, existing neural network processing relies on a centralizedtask scheduler that consumes significant processing and transactionaloverhead to coordinate between cores. In contrast, the sparse globalcommunications between cores of the exemplary multicore architecturedecouples neighborhood processing and enables the multicore architectureto asynchronously operate the cores in parallel. Consequently, anotheraspect described herein enables distributed task coordination betweencores and asynchronous handshaking protocols.

As previously alluded to, existing neural network processing istypically performed within the context of commodity servers and/orgeneral-purpose computers. In such contexts, processing bandwidth andmemory resources are typically managed by e.g., an operating system (OS)that centrally manages all of the concurrently running tasks for all ofthe compute resources (cores). Specifically, software processes areordered (re-ordered) and dispatched to a core for execution by acentralized task scheduler process. Typically, the task scheduler mustconstantly determine which task should be executed next based on e.g.,data path dependencies, priorities, and/or other system considerations.This requires a complex software-based scheduling program, supported atthe OS level, and by complicated, but generalizable low-level primitivessupport for synchronization and parallelization mechanisms e.g., mutexesand threading. Such solutions are impractical for embedded deviceoperation.

The localization techniques described herein decouple neighborhoods fromone another. Notably, the aforementioned neighborhood processing isentirely independent and makes up the majority of a core's processingload, whereas global connectivity is sparse and is asynchronouslybroadcast across all cores. Furthermore, the cores may vary widely incomputational load and may finish processing vectors at different times,etc. Consequently, another aspect of the present disclosure is directedto distributed asynchronous scheduling.

Notably, there are many different types of parallelism that may beleveraged in neural network processing. Data-level parallelism refers tooperations that may be performed in parallel over different sets ofdata. Control path-level parallelism refers to operations that may beseparately controlled. Thread-level parallelism spans both data andcontrol path parallelism; for instance, two parallel threads may operateon parallel data streams and/or start and complete independently.Various aspects of the present disclosure are directed to leveragingthread-level parallelism and asynchronous handshaking to decouplecore-to-core data path dependencies. More directly, the principlesdescribed herein enable threads to run independently of one another,without any centralized scheduling and/or resource locking (e.g.,semaphore signaling, critical path execution, etc.) Decoupling threaddependencies allows cores to execute threads asynchronously.

In one exemplary embodiment, the multicore architecture includes a setof distributed cores that run in parallel. The cores communicate witheach other via an interconnecting network of router nodes. Each coreprocesses its threads asynchronously with respect to the other cores.Most threads correspond to the dense neighborhood, and the core canprocess these threads independently of the other cores. Globalcommunication is sparse (infrequent) and is handled via an asynchronoushandshake protocol.

Exemplary Dependency-Triggered Execution

Various aspects of the present disclosure are directed to a multicorearchitecture that decouples processing among the cores and plans threadscheduling during compile-time. Thread scheduling in this manner greatlyreduces operating system (OS) complexity; in one exemplary embodiment,the compile-time thread scheduling relies on a “scoreboard” count thatobviates data path dependency-based thread scheduling at run-time.

FIG. 4 is a logical block diagram of an exemplary core's control anddata path operations. As shown therein, each core 400 of the multicorearchitecture includes: a scoreboard 402, a ready queue 404, aninstruction fetch/instruction decode (shown with lanes 406A and 406B), aprogram buffer 408, a router command input 410, an issue controller 412,router input/output 4141, 4140, and execution units and other sharedresources (memory, etc.)

In one embodiment, each core maintains its own scoreboard 402 to trackthread dependencies. The exemplary scoreboard 402 stores the “score” ofeach thread and its sleep/awake state. Each thread's score is a numberthat represents the number of thread dependencies (e.g. data, controldependencies on the actions performed by other threads) that must besatisfied before the thread can be executed. For example, at programstart, a given thread's score might be the number of other threads thatmust complete, each producing data that this thread depends on. When athread's score drops to zero (indicating that there are no furtheroutstanding dependencies), then the thread can be queued for executionvia the ready queue 404.

Notably, traditional scheduling techniques generally fall into twocategories: instruction scheduling, and thread prioritization.Instruction scheduling techniques dynamically schedule instructions (notthreads) for a processing pipeline so that instructions can execute outof order when there are no conflicts. In instruction schedulers, a datastructure (such as a table, scoreboard, etc.) logs the data pathdependencies for every instruction. Instructions are only released whenthe scheduler determines that there are no conflicts with previouslyissued and incomplete instruction. In contrast, thread prioritizationscheduling is often used by an operating system (OS), or similarcentralized entity, to schedule thread execution based on holisticsystem considerations (power, performance, available resources, etc.)Notably, OS schedulers operate at a level of abstraction above threadexecution; thus, OS schedulers don't consider (nor have visibility into)each thread's internal considerations. Unlike instruction scheduling andthread prioritization, the exemplary scoreboard 402 accounts fordependencies between threads (referred to throughout as “threaddependencies”). Thread dependencies identify the control and/or datainput and outputs to a thread, where each thread is a self-containedsequence of computer instructions.

Referring back to FIG. 4 , the exemplary ready queue 404 holds a runninglist of thread identifiers that are ready for execution. In oneexemplary embodiment, the ready queue 404 is a first-in-first-out (FIFO)buffer. Threads are enqueued by the scoreboard 402 when there are nooutstanding thread dependencies and are dequeued by the instructionfetcher/instruction decoder (IF/ID) to fill open lanes 406A, 406B (e.g.,opened when active threads sleep, complete execution, or otherwisetransition to an inactive state). Other implementations may use otherqueuing schemes including without limitation last-in-first-out (LIFO),priority queuing, round robin, shortest job first, and/or any otherqueuing scheme.

The instruction fetcher/instruction decoder (IF/ID) fetches and decodesthe instructions for active threads. Each lane 406A, 406B of the IF/IDhas an associated program counter (PC) that points to the nextinstruction to be executed. The IF/ID can support one thread for eachlane; a greater or fewer number of lanes may be substituted with equalsuccess, the two (2) lane implementation being purely illustrative.During operation, the instruction (opcode and one or more operands) atthe PC is fetched and decoded. After each instruction, the PC eitherincrements or jumps to the next instruction. Instructions are executeduntil a stop condition occurs (e.g., the thread completes, is put tosleep, etc.) When a thread completes, the thread's scoreboard and PC isreset. When a thread sleeps, the thread's return program counter iswritten to the program buffer 408 and the thread's score is set to theoutstanding thread dependencies.

The program buffer 408 stores sleeping threads' return program counters(PCs). The return program counter identifies the next instruction to beexecuted once the sleeping thread wakes up. As described in greaterdetail herein, a thread may need to put itself to sleep e.g., to waitfor other threads to asynchronously complete. A thread may suspend itsoperation by issuing a sleep instruction; the sleeping thread is removedfrom its lane 406A, 406B in the instruction fetch/instruction decode(IF/ID); the return program counter is stored in the program buffer 408,and its score in the scoreboard 402 is reset. When its score hasdecremented to 0, the thread is queued for execution via the ready queue404. The thread resumes execution in the IF/ID at the return programcounter stored in the program buffer 408.

The issue controller 412 assigns executable instructions from the IF/IDlanes 406A, 406B to an execution unit 416A, 416B, the scoreboard 402, orrouter input/output (I/O) 4141, 4140. In the illustrated embodiment, theexecution units 416A, 416B share a common resource (e.g., memory); thus,in some cases, the issue controller 412 may order instructions based onresource contention considerations. For example, the issue controller412 may check that both lanes 406A, 406B do not access the sameresource; in the event that a resource collision is present, the issuecontroller 412 may hold off one of the lanes. Other implementations mayuse a dedicated resource for each execution unit; such implementationsmay require more hardware but reduce issue controller logic (sincecollisions are not an issue).

The issue controller 412 may also arbitrate between the core's activethreads and instructions received via router command input 410. In somecases, inter-core instructions may be treated as an interrupt; forexample, inter-core instructions may interrupt ongoing thread execution.In other cases, intra-core instructions may take precedence overinter-core instructions; inter-core instructions may be held off untilintra-core instructions complete.

The exemplary router input/output (I/O) 4141, 4140 sends instructionsand data to other cores of the multicore of the multicore architecture.For example, a thread in one core may need to e.g., send sparseactivation vector data. The router I/O 4141, 4140 transacts data withother cores for use with the shared resources (e.g., stored in a memory,etc.) Additionally, inter-core instructions may be passed via theinter-core communications over the routing network. During inter-coreoperation, the router command input 410 receives instructions sent byother cores of the multicore architecture and parses (and/or generates)the appropriate operand/opcodes for core execution.

Execution units perform arithmetic operations (such as theaforementioned sparse vector-sparse matrix operations described supra.)Shared resources may include memory, registers, and/or any other logicthat may be shared and accessible by the execution units and router I/O4141, 4140.

Referring now to FIG. 5 , one exemplary thread dependency graph betweentwo (2) cores 500A, 500B is presented. As shown therein, a computationis implemented as a number of threads, where each thread may be composedof one or more instructions. The thread graph represents each thread asa node, and the edges of the graph describe the data path dependenciesbetween threads. For example, threads 504A and 506A of core 500A requiredata produced by thread 502A; the thread dependencies are representedwith a directional edge from thread 502A to threads 504A and 506A.Threads which have no outstanding dependencies may be run in parallel(e.g., threads 504A, 506A and 504B, 506B may be parallelized).

As a brief aside, thread dependency graphs are a useful tool to analyzerelationships between threads; historically, software designers havedesigned software based on thread dependency graphs. Such software maybe explicitly ordered at compile time (via compiler flags, etc.) so asto ensure that thread dependencies are preserved. In other words, thethread execution order is reflected in the static structure of thecompiled code, rather than dynamically determined at runtime accordingto instruction scheduling/thread prioritization. Notably however, suchtechniques assume the presence of a centralized task scheduler; in otherwords, the software designer has written the software such that acentralized scheduler executes the threads in the intended (structured)order.

As previously alluded to, neural networking loosely models theneuro-biological functionality found in the human brain. In biologicalsystems, each neuron operates independently of other neurons without acentralized management functionality or sequence of execution. Thus,neural networks are particularly well represented by asynchronous threaddependency graphs, e.g., each node may correspond to one or morethread(s). Various embodiments of the present disclosure use threaddependency graph analysis to decouple scheduling across many distributedcores when compiling a neural network model into hardware descriptionlanguage and/or software executables. In other words, rather than usingthread dependency graphs to generate a sequential ordering for acentralized scheduler that is evaluated at “run-time”, the individualthread dependencies define a count value for each thread at“compile-time.” Threads and their thread dependency count aredistributed to each core at “program-time”. In this manner, each corecan configure its dedicated control path logic for independentoperation. As shown in FIG. 5 , cores 0 and 1 have instructions andinitial scoreboard values for each of their constituent threads.Thereafter, each core can dynamically determine which threads to executebased on fulfilled thread dependencies without requiring a centralizedscheduler.

In one exemplary embodiment, threads are not structurally ordered,instead each core dynamically queues threads at runtime when thethread's dependencies are satisfied. In one such variant, each thread ispermanently placed within a single core for the lifetime of the program.The set of threads placed in each core is called a thread pool. Threadsin the same thread pool within one core share the same memory resources;for example, threads 502A, 504A, 506A and 508A may freely share data. Toshare data between different thread pools (between different cores),explicit inter-core communications are required (e.g., thread 508A doesnot share data with thread 502B, even though thread 502B depends onthread 508A). While the exemplary embodiments described hereinpermanently place threads and thread pools to cores, alternativeimplementations may allow e.g., cross-scheduling and execution ofthreads, and/or resource sharing across different cores. For example, athread may be suspended and migrated to a different core, etc. Variousother implementations may be substituted by artisans of ordinary skillin the related arts, given the contents of the present disclosure.

In one exemplary embodiment, each thread in the thread pool may changethe scoreboard's state. More directly, a first thread may produce datafor a second thread and then decrement the second thread's score to moveit closer to execution. Unlike existing techniques that rely on acentralized task scheduler, the exemplary scoreboard enables threadscheduling in a distributed manner. In other words, each core's threadsmay be dynamically increased or decreased by the ongoing threads (bothwithin the core, and external to the core). More directly, taskscheduling is incorporated within the threads themselves, rather than asa task scheduling process. Furthermore, the exemplary mechanismdescribed herein is embodied as hardware logic (the scoreboard 402),rather than instructions which are executed by a processor (asoftware-based process.) Hardware logic requires substantially lesscompute overhead and latency and can be implemented within embeddeddevices (low power, resource limited platforms.)

In the aforementioned embodiment, threads and thread pools shareresources of the core and are limited to cross-thread management withinthe same thread pool. To facilitate cross-thread management fordifferent cores (“inter-core thread management”), an asynchronoushandshake protocol enables access to scoreboards in the destinationcores. Consider one such exemplary communication sequence depicted inFIG. 5 ; ideally, both the sending thread 508A and a receiving thread502B are ready to communicate before inter-core data transmissionbegins. Without a guarantee that thread 502B is ready to receive data,thread 508A should not send anything over the routing network because itmay impede other inter-core communications. Similarly, the receivingthread 502B should not consume its core's shared resources while itwaits for the sending thread 508A.

In one exemplary embodiment, if the sending thread 508A has data to sendbefore recipient thread 502B is ready, then it goes to sleep. Therecipient thread 502B sends a notification to the sending thread 508A,that the recipient thread 502B is ready to receive data and then goes tosleep. The ready message travels over the interconnecting network ofrouter nodes and decrements the sending thread's score. When the sendingthread 508A wakes up and resumes operation, it starts streaming the dataover the interconnecting network of router nodes to the destinationcore's router I/O which stores the data into the destination core'sshared memory. When the stream has been completely transmitted, thesending thread 508A decrements the recipient thread's score. Therecipient thread 502B can wake and retrieve the data from the sharedmemory.

Example Operation

FIGS. 6A-6F provide a step through of an exemplary core's control anddata path operations corresponding to the core 0 thread pool operationsof FIG. 5 . While the following discussion is presented in the contextof single cycle pipelined instructions for ease and clarity ofillustration, the concepts described herein may be broadly extended tomulti-cycle instructions that are fully, partially, or not-at-allpipelined. Additionally, the exemplary cores may have many thread pools(not shown) in concurrent operation; the following logical eye diagramsprovide labeled “open eyes” to identify known logic states for theexemplary thread pool, and “closed eyes” to indicate don't care states(where the core may be idle or executing other threads).

Referring first to FIG. 6A, a first set of transactions 652 illustratehow the exemplary scoreboard controls multi-threaded operation usingthread dependencies. In this example, the scoreboard values areinitialized to:

$\begin{bmatrix}{{thread}0} \\{{thread}1} \\{{thread}2} \\{{thread}3}\end{bmatrix} = \begin{bmatrix}0 \\1 \\1 \\3\end{bmatrix}$

In this thread pool, only thread 0 is ready for execution; thus, threado enters the ready queue 604 in the second clock cycle and is assignedto an open lane 606A of the IF/ID starting at instruction 0 in the thirdcycle. Lane 606A reads the thread instructions and provides them to theissue controller 612 for issuance.

Transactions 654 depict to a multi-cycle asynchronous RDY handshake withanother thread of another core (core C_(j) thread T_(j)) to solicit newinput data. Since the data transfer is likely to take a long time,thread 0 immediately goes to sleep (SLEEP) by writing its currentinstruction state (3) in the program buffer (PB) 608. In this case, thesleep instruction (SLEEP) occurs before the other in-flight instructions(RDY, SB) have completed.

In transactions 656, the issue controller 612 continues to monitor andmanage its in-flight instructions for lane 606A. For instance, the issuecontroller queues a scoreboard write instruction (SB) to occur after therouter output 4140 completes its RDY handshake; once the RDY handshakeis complete, the SB instruction increments the scoreboard value ofthread 0 by 1. At this time, the scoreboard values for this thread poolare:

$\begin{bmatrix}{{thread}0} \\{{thread}1} \\{{thread}2} \\{{thread}3}\end{bmatrix} = \begin{bmatrix}1 \\1 \\1 \\3\end{bmatrix}$

Even though core 0's own thread pool is inactive, external threaddependencies can affect core 0's operation. Referring now totransactions 658 of FIG. 6B, core C_(j) thread T_(j) responds to core0's RDY handshake. As shown in FIG. 6B, external control pathcommunications are received via the router command input 610. In theillustrated embodiment, the external control path may enable directaccess to the issue controller 612. In other implementations, theexternal control path may trigger execution of a special-purposeinter-core thread that implement core-to-core specific functionality.For example, core-to-core communication may need to be authenticated,encrypted/de-crypted, verified, validated, or otherwise checked beforeexecution. In other such examples, the special-purpose thread may handlecore-specific optimizations; for example, the thread may wake-up,initialize, and/or warm-up portions of the core. This may beparticularly useful for devices with multiple power management states(low power, light sleep, deep sleep, etc.) Still other variants may besubstituted by artisans of ordinary skill in the related arts, given thecontents of the present disclosure.

Referring back to transactions 658, core C_(j) thread T_(j) writes datainto the shared resource 618 via router input 614I. The issue controller612 monitors the multi-cycle transaction between the shared resource 618and the router input 614I and issues a scoreboard update to thread 0when the input data is fully received. At this point, the scoreboardvalues are:

$\begin{bmatrix}{{thread}0} \\{{thread}1} \\{{thread}2} \\{{thread}3}\end{bmatrix} = \begin{bmatrix}0 \\1 \\1 \\3\end{bmatrix}$

Based on its thread dependency count, thread 0 can enter the ready queue(RQ) to resume operation. Once again, thread 0 is assigned to the firstopen lane which is IF/ID lane 606A (transaction 660 of FIG. 6C). Thread0's next operations are to: (i) decrement the scoreboard for both thread1 and thread 2 (transaction 662 of FIG. 6C), and (ii) jump back to waitfor more input data (dashed transaction 664 mirrors the earliertransaction 654). At the conclusion of transaction 662, the scoreboardis:

$\begin{bmatrix}{{thread}0} \\{{thread}1} \\{{thread}2} \\{{thread}3}\end{bmatrix} = \begin{bmatrix}0 \\0 \\0 \\3\end{bmatrix}$

FIG. 6D presents eye diagram representations of concurrent execution ofthread 0, thread 1, and thread 2, in view of the scoreboard (SB) 602,ready queue (RQ) 604, and program buffer (PB) 608 contents. As shown inFIG. 6D (and also previously noted in FIG. 6C), IF/ID lane 606A isexecuting thread 0; thread 0 has jumped back to wait for more data(transactions 664). At the same time, while thread 1 and thread 2 bothhave a thread dependency count of 0, only lane 606B is available.Consequently, thread 1 immediately enters the IF/ID, however thread 2must wait in the ready queue (RQ) until a lane opens up (transactions666). In this case, when thread 0 completes (end of transactions 664) itgoes to sleep which frees IF/ID lane 606A for thread 2 (end oftransactions 666).

As shown in transactions 668, thread 1 is assigned to IF/ID lane 606B.Notably, the issue controller 612 is initially occupied with other tasks(the SB instruction of thread 0) so thread 1's operation (OP) is delayedin-queue before being issued to the first available execution unit(execution unit 616A). The issue controller 612 monitors progress of theexecution unit 616A and performs two SB updates thereafter (decrementingthread 3's thread dependency by 1 and incrementing its own threaddependency count by 1.) As previously noted, thread 1 is allowed tosleep as soon as its instructions are in-flight.

Referring now to transactions 670, thread 2 is assigned to the nextavailable IF/ID lane (lane 606A) and the next available execution unit(execution unit 616B). As with thread 1, the issue controller 612monitors progress of the execution unit 616B and performs two SB updatesthereafter (decrementing thread 3's thread dependency by 1 andincrementing its own thread dependency count by 1.) Thread 2 is alsoallowed to sleep as soon as its instructions are in-flight. At theconclusion of transaction 670, the core can switch to other tasks or itcan drop into a low power state because there are no threads waiting inthe ready queue (RQ). In summary, the scoreboard at this point is:

$\begin{bmatrix}{{thread}0} \\{{thread}1} \\{{thread}2} \\{{thread}3}\end{bmatrix} = \begin{bmatrix}1 \\1 \\1 \\1\end{bmatrix}$

At a later point, another thread of another core can request thecalculated data from core 0 by issuing a RDY instruction anddecrementing thread 3's thread dependency count (FIG. 6E, transactions672). The scoreboard at this point is:

$\begin{bmatrix}{{thread}0} \\{{thread}1} \\{{thread}2} \\{{thread}3}\end{bmatrix} = \begin{bmatrix}1 \\1 \\1 \\0\end{bmatrix}$

In FIG. 6F, the core 0 thread 3 is assigned to the first available IF/IDlane (lane 606A). The issue controller 612 assigns the first availableexecution unit (execution unit 616A) to perform an arithmetic operation(OP) on the shared resource 618 (transactions 674). The issue controller612 monitors the OP progress; when OP completes, the issue controllersends the data in the shared resource 618 via the router I/O (6140) andupdates its scoreboard (transactions 676):

$\begin{bmatrix}{{thread}0} \\{{thread}1} \\{{thread}2} \\{{thread}3}\end{bmatrix} = \begin{bmatrix}1 \\1 \\1 \\3\end{bmatrix}$

As can be appreciated from the foregoing discussion of FIGS. 6A-6F,managing thread dependencies using the core's dedicated control pathdata structures (scoreboard 602, ready queue 604, and program buffers608) ensures that each core can asynchronously and independently manageits own instruction execution without centralized task scheduling. Thecontrol path logic only queues threads that have satisfied their threaddependencies. Since the thread dependency counts were determined fromthe logical structure of the thread graph at compile-time, the controlpath logic implicitly (structurally) prevents run-time hazards withoutincurring additional scheduling overhead.

Methods

Referring now to FIG. 7 , one logical flow diagram of a generalizedmethod 700 for thread-based scheduling is shown. The generalized methodmay be executed by a core of a multicore processor (such as cores ofFIG. 4 and FIGS. 6A-6F) that includes dedicated control path logic formanaging threads e.g., a scoreboard, a ready queue, and a programbuffer. In the exemplary implementation, the scoreboard indicates whichthreads have satisfied their thread dependency counts and can be queuedfor execution, the ready queue orders the threads for execution, and theprogram buffer identifies the current/next instruction for each thread.

At step 702, thread states are monitored in control path logic. In oneaspect, the control path logic is dedicated for thread control. As abrief aside, the “control path” refers to logic and/or functionalitythat affects the sequence of operations that are performed and/or thatare fetched to be performed (“in-flight” instructions). Typical examplesof control path operations include without limitation: jumping,branching, and/or conditional execution. The “data path” refers to logicand/or functionality that manipulate addressable data. As a practicalmatter, addressable data is shared across multiple threads and/or cores;thus, run-time data path dependencies may result in data access hazards.Notably, some control path operations may share the data path resourcesand/or vice versa; for example, control path instructions may use“intermediate” values that are stored in addressable data (e.g.,intermediate format, or I-format). However, other control pathoperations have dedicated logic that is outside of the data path; forexample, internal processor registers are not addressable via theaddress bus (e.g., so-called register format, or R-format instructions).

One salient advantage of using dedicated control path logic for managingthread execution is that it ensures that each core can determine whichthread to execute next without blocking other threads' and/or cores'access to resources (e.g., a shared memory). More generally, however,any scheme for allowing nodes to independently and/or asynchronouslyoperate within a neural network may be substituted with equal success.For example, hardware implementations of a neural network may utilizecontrol logic circuitry to ensure that node dependencies are preserved(using gated enable/disable logic to control the data path, rather thansequencing thread execution). Similarly, software implementations of aneural network may utilize logical semaphores and/or other datastructures (e.g., signaling protocols, handshaking mechanisms, etc.)

In one aspect, the control path logic is determined at compile-time. Forexample, neural network software may use threads to describe thefunctionality of each node of the network. The compiler generates athread dependency graph by identifying data dependencies between threadsand translates the functionality of each node into machine code(opcodes) at “compile-time.” Once compiled, the instructions and threaddependency counts may be stored into a non-transitory computer readablemedium for a processor core to execute during “run-time” execution.

By direct extension, artisans of ordinary skill in the related artsgiven the contents of the present disclosure will readily appreciatethat the techniques described herein are broadly applicable to any formof hardware and/or circuit logic that can benefit from structured datadependency avoidance at compile-time (versus checking data dependency atrun-time). Notably, a field-programmable gate array (FPGA) implementshardware description language (HDL) within look-up-tables (LUTs) andsequential logic (registers); application specific integrated circuits(ASICs) implement HDL within combinatorial gates (AND, OR, NAND, NOR,etc.) and sequential logic. While FPGAs and ASICs may be flexiblydesigned to handle large data busses, this takes more silicon realestate and incurs proportional increases in power-consumption andperformance penalties for each bit of data. Consequently, compilingnodes to structurally avoid data dependencies reduces both the amountand complexity of the resulting network.

Various embodiments of the present disclosure monitor a thread state. Inone embodiment, thread state corresponds to a count of threaddependencies. As a brief aside, a “thread” is the smallest discrete unitof processor utilization that may be scheduled for a core to execute. Athread is characterized by: (i) a set of instructions that is executedby a processor, (ii) a program counter that identifies the current pointof execution for the thread, (iii) a stack data structure thattemporarily stores thread data, and (iv) registers for storing argumentsof opcode execution. A “single-threaded” application consists of onlyone thread whereas a “multi-threaded” application may have a distinctprogram counter, stack, and set of registers for each thread. In somevariants, the multi-threaded application may share resources (e.g., acommon code base, memory resource, etc.) In other variants, one or morethreads of the multi-threaded application may have dedicated resources.

As used herein, the terms “state” and/or “stateful” refer to threadmanagement techniques that maintain an ongoing status to execute atransition; a transition is a set of actions to be executed when acondition is satisfied and/or event occurs. Stateful thread managementimplements control path logic within the core itself to track eachthread's state (i.e., without relying on centralized task schedulingsoftware).

In one embodiment, thread state may include the thread dependencies thatare necessary to unblock execution. As used herein, a thread is“blocking” (or “blocks” other threads) when its execution causes ahazard for at least one other thread (its thread dependencies). Commonexamples of hazards include data hazards, structural hazards, andcontrol hazards. Hazards are well understood by artisans of ordinaryskill in the computing arts and not further discussed herein. An“unblocked” thread has no current dependencies, i.e., its threaddependencies (if any) have been satisfied.

In one embodiment, thread state may include the execution progress of acurrently blocked thread. In one such implementation, a program buffermay store the next (or current) instruction in the thread. Other typesof execution progress that may be stored may include e.g., registercontents, operands, wake-up interval/sleep-duration, and/or any otherthread-specific information. While the foregoing examples are presentedin the context of a thread that is blocked by a data dependency,artisans of ordinary skill in the related arts given the contents of thepresent disclosure will readily appreciate that a variety of otherdependencies may affect thread state. For example, thread state may bebased on resource and/or environmental dependencies. As but one suchexample, a thread may be “woken” when a sensor detects activity ofinterest (e.g., a triggering sound, image, etc.) In other examples, athread may sleep when resources (processing cycles, memory, networkbandwidth, hardware logic, etc.) are unavailable, and wake-up when theresource is free. Still other implementations may hierarchically enablenetworks of increasing complexity based on stimulus dependencies; forexample, a short stimulus (e.g., a “wake word”) may only excite a fewthreads, more complex stimulus (e.g., a spoken sentence) may trigger alarger neural network with commensurately more threads. Otherimplementations may be substituted with equal success.

While each core operates independently of other cores, the techniquesdescribed herein enable threads within each core to affect the threadstate of other threads in other cores. The lack of centralizedmanagement and highly flexible connectivity means that the thread stateof a core's threads may change without any of the core's own activity.In one embodiment, each core independently monitors its threads toidentify threads that are ready for execution. In other embodiments, acore may be notified of changes to its thread states. For example, anexternal thread (running in another core) may change the scoreboard fora core's internal threads. The external thread may additionally flag thestate change to the core. In this manner, the core may only infrequentlyneed to be checked. More generally, a core may monitor threadscontinuously, periodically, when instructed to do so, on an as-neededbasis (e.g., when an IF/ID lane, or other processing resource frees up),or any other frequency.

More broadly, various aspects of the present disclosure are directed towait-free access to thread state for multiple concurrently operatingthreads. Here, “wait-free” refers to non-blocking access on a per-threadbasis. While the foregoing discussion is presented in the context ofcontrol path logic for a core of a multicore architecture, thetechniques broadly enable many different architectures to performmulti-threaded operations without fear of hazards; this functionality isparticularly useful for asynchronous (no shared time reference) andindependent (no centralized coordination) parallel processing. Thetechniques described herein may be extended for a network of processingcores that are separated from one another, potentially by substantialdistance or time. For example, a large neural network may be split intothread pools for distribution within a server farm or potentially evenacross wired/wireless networks. In other examples, a neural network maydo computations in idle time (e.g., unpredictable gaps of time betweenthread executions). In other words, by de-coupling threads andimplementing a wait-free queuing system, any number of nodes may beimplemented subject only to time, power and/or memory constraints.

While the present disclosure uses dedicated control path logic (e.g.,registers and/or combinatorial logic in a processing core), anytechnique for wait-free monitoring of thread state may be substitutedwith equal success. For example, more complex implementations may use adistributed scoreboard that is accessible from multiple machines andlocally cached. The locally cached scoreboard may be updated by eachmachine as their threads complete; when the locally cached scoreboardneeds to update other threads, the updates may be transferred out. Whilethis scheme is substantially more complex than the dedicated controlpath logic implementation, artisans of ordinary skill given the contentsof the present disclosure will readily appreciate that it could providewait-free access on a substantially larger scale. This may be useful forother applications (e.g., very large neural networks that have relaxedtime constraints, etc.)

Referring back to FIG. 7 , when a thread state is ready for execution,the thread is assigned resources (steps 704 and 706). In someimplementations, resource utilization is monitored and assigned whenavailable (step 708).

In one aspect, the unblocked threads are assigned a priority ofexecution based on their order in a ready queue. In one exemplaryembodiment, the next ready thread is assigned to the first availableinstruction fetch/instruction decode (IF/ID) lane. The IF/ID laneresumes thread operation at the next instruction (based on thread stateinformation stored within the program buffer). In one specificimplementation, the thread's IF/ID lane assignment persists until thethread suspends itself. In other implementations, the thread's IF/IDlane assignment may be released or revoked; for example, the issuecontroller may revoke the lane assignment if the thread's resources areunavailable, similarly, the thread's instructions may be structured asbranch statements (e.g., if resource is available issue, otherwisesuspend). Still other techniques common in the processing arts may besubstituted with equal success.

While the foregoing discussion is presented in the context of afirst-come-first-served basis (e.g., a first-in-first-out (FIFO) queue),other priority schemes and ready queues may be used with equal success.As but one such example, the ready queue may be implemented as alast-in-first-out (LIFO), or first-in-last-out (FILO), ready queue.Certain types of threaded operations may be more efficiently handledwith a LIFO stack; for example, pushing a thread into the LIFO readyqueue ensures that all subsequent threads will be handled first (e.g.,at a higher priority). In still other implementations, the ready queuemay be prioritized or re-ordered; for example, certain threads may beprioritized for execution as soon as they are unblocked and/orde-prioritized to execute when there are no other un-blocked threads(“best effort” execution). Other prioritization techniques may offerfairness or load balancing; examples of queuing techniques include,without limitation: round robin, weighted round robin, water-filling,etc.

In another aspect, thread instructions may be assigned processingresources. In one specific implementation, the issue controller tracksthe utilization of the processor core's resources, e.g., the executionunits during multi-cycle data manipulations and router input/output(I/O) during data transfers. In some embodiments, instructions areassigned to processor resources on a first-come-first-served basis.Other schemes may be also be used; for example, resources may beassigned to keep one execution unit fully loaded, and another executionlightly loaded (if at all). This may be beneficial from powerconsumption standpoints. Still other embodiments may load balance tomaximize overall device performance.

While the foregoing examples are presented in the context of symmetricprocessing resources, asymmetric capabilities of different processingresources may also be considered by the issue controller. For example,certain execution units may be better suited to low power operation(e.g., lower clock rates, reduced width data busses, etc.) whereas otherunits may be high performance (e.g., higher clock rates, wider databusses, etc.) Similarly, while the foregoing scheme is presented in thecontext of a broadcast-based router input/output, other implementationsmay use multicast and/or unicast router I/Os. This may be preferable foroperations and/or communications that have benefit from high bandwidthand/or low fan-out.

It is appreciated that most neural networking applications operatewithin the context of a broader system, thus a variety of differentresources may be assigned by an issue controller. Examples of suchresources include, without limitation: encryption/decryption engines,security modules (authentication, authorization, ciphering, etc.),encoding/decoding (codec) for audio and/or visual media, digital signalprocessing (DSP), and/or any variation of dedicated arithmeticoperations. As but one such example, a natural language processingmachine learning application may use code-excited linear prediction(CELP) resources to perform speech coding, which is then processedwithin a neural network pool. In some cases, the natural languageprocessing may additionally be obfuscated or otherwise encrypted toprevent unintended data leakage (or other security and privacybreaches). As another such example, a multicore network that spansmultiple devices may need to incorporate secure communication to ensurethat the communications are not maliciously attacked or snooped intransit.

Artisans of ordinary skill in the related arts will readily appreciatethat resource assignments may be based on factors other than resourceutilization. Other considerations may include e.g., application-specificconsiderations (e.g., latency, throughput, accuracy, robustness, powerconsumption, etc.), device-specific limitations (e.g., processingcycles, memory space, network bandwidth, power and/or clock limitations,etc.), etc. For example, cloud-based neural networks have access tonearly infinite cloud compute resources however compute time may becharged on thread utilization; in such cases, threads may be compiled atcompile-time to reduce overall thread utilization (e.g., collapsingmultiple related threads into a single thread, etc.) In another suchexample, time insensitive applications may have a limitless time budgetfor the neural network and should be preferably performed on idleresources. In such cases, threads may be compiled at compile-time tosupport a wide gamut of potential processing resources.

In some cases, thread execution may consume a resource for multiplecycles; thus, thread execution within the assigned resources may bemonitored for completion (step 708). Yet other implementations may havea known or deterministic cycle time; for example, an instruction orresource may specify that execution will complete within a number ofcycles, a length of time, etc. Other implementations may use completionsignaling or other protocol to notify the issue controller when theexecution is complete; such techniques are typically used where theutilization is indeterminate and/or indeterminable by the core. Forexample, a handshake protocol can be used to indicate when a routertransaction is complete. More broadly, artisans of ordinary skill in therelated arts will readily appreciate that execution progress ismonitored to avoid potentially blocking behavior (e.g., multipleinstructions that would attempt to access the same execution unit,shared resource, etc.) In other words, the issue controller can monitorresource utilization to prevent resource hazards.

In other cases, resources may be single-cycled, non-branching, orotherwise amenable to feed-forward pipelining; the issue controller can“fire-and-forget” in such implementations. For instance, theillustrative examples described above describe sleep instructions thatwrite the current thread pointer to a program buffer and set a threaddependency count for the thread in the scoreboard. Pipelined operationgenerally improves processor performance and/or reduces overall systemlatency.

Finally, thread states are updated in control path logic (step 710). Inone embodiment, a thread updates its own thread state or a thread stateof a thread in its thread pool. In some such variants, the thread stateupdates may occur synchronously via an executed instruction. In otherembodiments, a thread may update other thread states in other threadpools, cores, devices, or even multicore networks. In some suchvariants, the thread state updates may occur asynchronously via an amulti-cycle handshake or similar asynchronous signaling protocol.

Notably, various embodiments described herein implement a machinelanguage instruction (“opcode”) that updates a thread dependency countin accordance with a thread dependency graph determined by the compiler.More broadly, various aspects of the present disclosure are directed toa compiler generated instruction that updates thread states inaccordance with a thread state graph that is not determined at run-time.As used herein, the term “graph” refers to a logical structure that iscomprised of vertices (or nodes) and edges (links). Edges may bedirectional or direction-less. Graph analysis structurally constrainsthread execution because a thread may only execute when its dependencieshave been satisfied. Graph analysis greatly reduces the complexity ofthread scheduling and allows for distributed scheduling across a networkof independent nodes. While the exemplary embodiments are described withrespect to a scoreboard-based thread queuing, artisans of ordinary skillin the related arts will readily appreciate that any implementation of astructural graph may be used to control thread execution. As but onesuch example, finite state machines may be used to control threadexecution.

It will be appreciated that the various ones of the foregoing aspects ofthe present disclosure, or any parts or functions thereof, may beimplemented using hardware, software, firmware, tangible, andnon-transitory computer-readable or computer usable storage media havinginstructions stored thereon, or a combination thereof, and may beimplemented in one or more computer systems.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the disclosed embodiments ofthe disclosed device and associated methods without departing from thespirit or scope of the disclosure. Thus, it is intended that the presentdisclosure covers the modifications and variations of the embodimentsdisclosed above provided that the modifications and variations comewithin the scope of any claims and their equivalents.

What is claimed is:
 1. A multicore processor, comprising: a first coreand a dedicated first control path logic configured to store a first setof thread states; a second core and a dedicated second control pathlogic configured to store a second set of thread states; at least onerouter configured to asynchronously connect the first core to the secondcore, and where the first core comprises at least one instruction, thatwhen executed by the first core, synchronously updates the first set ofthread states of the first core, independently of the second core. 2.The multicore processor of claim 1, where the second core comprises atleast one second instruction, that when executed by the second core,asynchronously updates the first set of thread states of the first core.3. The multicore processor of claim 2, where the first core comprises atleast one third instruction, that when executed by the first core,asynchronously updates the second set of thread states of the secondcore.
 4. The multicore processor of claim 2, where the first set ofthread states correspond to a first set of nodes of a neural network andthe second set of thread states correspond to a second set of nodes ofthe neural network, and where the first set of nodes and the second setof nodes of the neural network operate concurrently.
 5. The multicoreprocessor of claim 4, where the first set of thread states and thesecond set of thread states correspond to a thread dependency graph ofthe neural network determined at compile-time.
 6. The multicoreprocessor of claim 4, where the dedicated first control path logiccomprises a first scoreboard, a first ready queue, and a first programbuffer, and where the dedicated second control path logic comprises asecond scoreboard, a second ready queue, and a second program buffer. 7.A method for communicating between multiple cores of a multicoreprocessor, comprising: storing a first set of thread states for a firstcore within a dedicated first control path logic; storing a second setof thread states for a second core within a dedicated second controlpath logic; asynchronously routing a first plurality of packets betweenthe first core and the second core, and where the first coresynchronously updates the first set of thread states and the second coresynchronously updates the second set of thread states.
 8. The method ofclaim 7, further comprising asynchronously routing a second plurality ofpackets between the first core and a third core.
 9. The method of claim8, where the first plurality of packets and the second plurality ofpackets are addressed to their respective destination cores.
 10. Themethod of claim 7, where at least one packet of the first plurality ofpackets writes to a scoreboard of the dedicated first control pathlogic.
 11. The method of claim 10, where the first set of thread statescorrespond to a first set of threads, and where at least one otherpacket of the first plurality of packets writes to an addressable memorythat is shared by the first set of threads.
 12. The method of claim 11,further comprising identifying a first thread of the first set ofthreads that is ready for execution based on a first thread state. 13.The method of claim 7, where the first core and the second core use ahandshake protocol to asynchronously route the first plurality ofpackets.
 14. The method of claim 13, where the handshake protocolprovides wait-free access to the first set of thread states and thesecond set of thread states.
 15. A multicore processor, comprising: aplurality of cores, where each core comprises dedicated control pathlogic configured to manage execution of a plurality of threads by thecore and a shared memory resource that is accessible by the plurality ofthreads; at least a first router configured to route a plurality of datapackets between at least a first core and a destination core, and wherethe first core comprises at least one instruction, that when executed bythe first core, causes the first router to route a data packet thatupdates one or more thread states of a destination dedicated controlpath logic of the destination core.
 16. The multicore processor of claim15, where the first core comprises the first router, and the destinationcore comprises a second router in communication with the first router.17. The multicore processor of claim 15, where the first core comprisesthe first router, and the destination core comprises a destinationrouter in communication with an interconnecting network of routers. 18.The multicore processor of claim 15, where a first set of threads of thefirst core correspond to a first node of a neural network, a second setof threads of the destination core correspond to a second node of aneural network, and where the first core and the destination coreconcurrently execute the first set of threads and the second set ofthreads.
 19. The multicore processor of claim 18, where a first set ofthread states corresponding to the first set of threads and a second setof thread states corresponding to the second set of threads correspondto a thread dependency graph of the neural network generated atcompile-time.
 20. The multicore processor of claim 15, where thededicated control path logic comprises a scoreboard, a ready queue, anda program buffer.